1. Field of the Invention
This invention relates to fenestration assemblies for both new and retrofit construction and in particular to energy efficient fenestration assemblies.
2. Background
Over the past forty years, the energy efficiency of windows has been significantly improved. One key technological improvement has been the development of low-emissivity coatings with sputtered low-e coatings offering the highest performance. Generally, there are two main types of sputtered coatings: solar control and solar gain. Comparing the two coatings, the emissivity of the solar control coating is lower resulting in reduced heat loss. However in a comparison study by the National Research Council of Canada using side-by-side test house monitoring, the study showed that with solar gain low-e coatings overall building energy consumption is 10 percent lower because of higher direct solar gains during the heating season. Although during the cooling season because of higher solar gains, building energy consumption is higher with solar gain low-e coatings.
Typically because of durability concerns, high performance sputtered coatings have to be located on the cavity glass surfaces of an insulating glass unit. However recently, more durable sputtered low-e coatings have been developed that can be used on exterior surfaces and by adding an exterior low-e coating to the outer interior surface of a double glazed unit, center-of-glass insulating performance is typically increased from R-4 to R-5.
Vacuum insulating glass (VIG) is an energy efficient window product that can provide outstanding center-of-glass insulating performance. With vacuum insulating glass, there is minimal heat loss through convection or conduction across the small vacuum cavity and the main heat loss source is through radiation. By using an ultra low emissivity coating, radiation heat loss can be reduced to a minimum and this can provide for R-15 center-of-glass performance for a double-glazed unit. However with a high performance solar control coating on surface two (glazing surfaces numbered from the exterior), direct solar heat gains through south-facing windows can be substantially reduced during the heating season and this lowers overall window energy performance.
As well in order to maintain the vacuum within the VIG unit, the two glass sheets are fused together at the edge resulting in a substantially lower R-value around the perimeter edge, for example about R-1. If the VIG unit is installed in a conventional window frame, R-value performance is further downgraded and so despite the impressive center-of-glass R-value performance, overall window performance is not substantially higher than with a conventional double glazed window.